1. Technical Field
This disclosure generally relates to electromagnetic methods and tools for well logging, and more particularly to improved antennas and methods for synthesizing antenna response.
2. Description of the Related Art
The measurement of dielectric constant (or dielectric permittivity) of formations surrounding a borehole is known to provide very useful information about the formations. The dielectric constant of the different materials of earth formations vary widely (for example, 2.2 for oil, 7.5 limestone, and 80 for water), so measurement of dielectric properties is a useful means of formation evaluation. For example, if the lithology and the water saturation of a particular formation are known, then the porosity may be determined if the dielectric constant of the formation could be obtained. Similarly, if the lithology and porosity are known, information as to the degree of water saturation can be obtained by measuring the dielectric constant of the formation.
A logging device that improved the art of measuring formation dielectric constant was the electromagnetic propagation tool as disclosed, for example, in the U.S. Pat. No. 3,944,910 (“the '910 patent”) issued to Rau and assigned to the present assignee. This patent discloses a logging device including a transmitter and spaced apart receivers mounted in a pad that is urged against the borehole wall. Microwave electromagnetic energy is transmitted into the formations, and energy which has propagated through the formations is received at the receiving antennas. The phase shift and attenuation of the energy propagating in the formations are determined from the received signals. The dielectric constant and, if desired, the conductivity of the formations can then be derived from the phase shift and attenuation measurements.
The configuration of antennas is an important aspect of successful operation of electromagnetic propagation logging tools. At a relatively high frequency of operation (for example, 1100 MHz), the signal attenuates quite rapidly. Therefore, it is important to have transmitting antennas that can efficiently transmit energy into the formations, and to have receiving antennas that can efficiently receive energy that has propagated through the formations. Because the accuracy of the dielectric constant and conductivity measurements depends upon accurate measurements of attenuation and phase shift of the received signals, it is essential that the antennas operate in a stable manner over time and that the antennas are, and remain, in a substantially balanced condition.
In the '910 patent, the antennas in the electromagnetic propagation logging device are cavity-backed or slot antennas, that are filled with a dielectric material and include a probe that is an extension of the center conductor of a coaxial cable. The center conductor of the coaxial cable, also known as the “probe”, extends across the slot and connects to the wall on the opposite side of the slot. The probe of the antenna, as disclosed in the '910 patent, extends in a direction parallel to the longitudinal direction of the borehole. This configuration is known as a “broadside” array. U.S. Pat. No. 4,704,581 (“the '581 patent”), issued to Clark and assigned to the present assignee, discloses a similar logging device, but wherein the antennas have probes that extend in a direction perpendicular to the longitudinal direction of the borehole. This configuration is known as an “endfire” array. The endfire array exhibits a deeper depth of investigation and is less affected by tool standoff (e.g., from mudcake) than the broadside array. On the other hand, the broadside array exhibits a stronger signal than the endfire array and may be preferred in relatively lossy (low resistivity) logging environments.
An example of a logging device based on the teachings of the '910 and '581 patents is an electromagnetic propagation tool sold under the trade name of EPT™ by Schlumberger Technology Corp. (Houston, Tex.). A similar tool, called adaptable EPT™ (“ADEPT™”), can provide either broadside operation or endfire operation during a given run, depending on the antenna selection. The ADEPT™ logging tool has two changeable antenna sets, one a broadside antenna array and the other an endfire antenna array. The EPT™ or ADEP™ tools use cavity-backed antenna (or slot antenna) arrays. Other related tools based on similar arrays include U.S. Pat. No. 4,698,572 (“the '572 patent”) issued to Clark. The '572 patent discloses electromagnetic logging tools incorporating slot antennas that have improved properties as compared with the conventional cavity-backed antennas. The slot antennas disclosed in this patent include tuning elements to improve their operation.
Furthermore, U.S. Pat. No. 5,434,507 (“the '507 patent”) issued to Beren et al. discloses electromagnetic logging tools with two-dimensional antenna arrays. The antenna arrays may comprise slot antennas having two conductors arranged in a crossed configuration. Such an antenna is known as a cross dipole antenna. The two-dimensional array of antennas makes it possible to image the formations surrounding the borehole.
More recently, printed circuit antennas have been disclosed in U.S. Patent Application Publication No. 2006/0145700 (“the '700 application”) in the name of Tabanou and assigned to the present assignee. These antennas may include printed circuit loops formed on an insulating layer. The loops may be configured to simulate the current paths of the conventional slot antennas noted above. The antenna has a reduced profile and more flexible structure, making it less prone to break under stress and therefore particularly useful in relatively harsh logging-while-drilling (LWD) applications.
While the above described antennas have been very reliable for obtaining electromagnetic logging information, the accuracy of that information has been limited by the mixed mode response generated by conventional antennas. Different antenna designs have different radiation characteristics that may be beneficial or detrimental to certain formation measurements. In dielectric and resistivity logging, for example, it is desirable to have high vertical resolution and low sensitivity to the standoff layer (i.e., mudcake) present between the antenna array and the formation. An antenna having a purely transverse electric (TE) radiation pattern would be ideal for standoff immunity, while an antenna having a purely transverse magnetic (TM) radiation pattern would be ideal for improved vertical resolution. Currently, however, there is no practical antenna design for downhole application that achieves pure TE or pure TM modes. An endfire array has mostly, but not completely, TE response while a broadside array has mostly, but not completely, TM response. In general, any antenna response can be decomposed into a mixture of TE and TM responses.
Accordingly, it is desirable to have a method of synthesizing pure TE and TM responses from the non-ideal, mixed response of existing antennas, and to have antenna arrays particularly adapted to provide the responses used in such synthesis.